btrfs: qgroup: avoid memory allocation if qgroups are not enabled

At btrfs_qgroup_inherit() we allocate a qgroup record even if qgroups are
not enabled, which is unnecessary overhead and can result in subvolume
creation to fail with -ENOMEM, as create_subvol() calls this function.

Improve on this by making btrfs_qgroup_inherit() check if qgroups are
enabled earlier and return if they are not, avoiding the unnecessary
memory allocation and taking some locks.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>